Nonvolatile semiconductor memory device

ABSTRACT

A nonvolatile semiconductor memory device according to one embodiment of the present invention includes: a memory cell array and a control circuit. The control circuit executes a first reading operation and a second reading operation. The first reading operation is an operation of reading a threshold voltage set in the selected memory cell by setting a voltage between a control gate electrode and source of the selected memory cell to a first value. The second reading operation is an operation of reading a threshold voltage set in the selected memory cell by setting a voltage between the control gate electrode and source of the selected memory cell to a second value lower than the first value. When executing the second reading operation, the control circuit keeps a voltage of the control gate electrode of the selected memory cell to 0 or a positive value.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 13/246,004, filed Sep. 27, 2011and is based upon and claims the benefit of priority from prior JapanesePatent Application No. 2011-84762, filed on Apr. 6, 2011, the entirecontents of which are incorporated herein by reference.

BACKGROUND

1. Field

Embodiments described herein relate to an electrically-rewritablenonvolatile semiconductor memory device.

2. Description of the Related Art

A NAND type flash memory has a memory cell array which is configured byarranging memory strings each including a plurality of memory cellsconnected in series. Both ends of each memory string are connected to abit line and a source line through select transistors respectively. Thecontrol gate electrodes of the memory cells of each memory string areconnected to different word lines respectively. In each memory string,the plurality of memory cells are connected in series with sources anddrains shared between them. The NAND type flash memory can have a smallunit memory cell size, because select transistors and their bit linecontacts and source line contacts are shared among a plurality of memorycells. Further, the NAND type flash memory is suitable forminiaturization, because the word lines and the device regions of thememory cells have a shape that resembles a simple stripe shape, andhence a flash memory having a large capacity is realized.

As the miniaturization of NAND type flash memories progresses,interference between adjoining cells and influence due to elapse of timeafter data writing increase, which might change the memory cell data.For example, when data written in a memory cell remains un-accessed fora long time, there occurs a phenomenon that electrons are dischargedfrom the charge accumulation layer of the memory cell, changing thethreshold voltage of the memory cell to a lower value. Hereinafter, thisphenomenon will be referred to as deterioration of data retention. Ifdeterioration of data retention occurs, a data reading operation mayfail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a schematic configuration of anonvolatile semiconductor memory device according to an embodiment.

FIG. 2 is a circuit diagram showing a configuration of a memory cellarray 1 of a nonvolatile semiconductor memory device according to anembodiment.

FIG. 3 is a diagram showing an example of data storage in a flash memoryfor storing four values.

FIG. 4 is a diagram explaining threshold voltage distributions andreading voltages of a reading operation according to a comparativeexample.

FIG. 5 is a flowchart showing a data reading process according to thecomparative example.

FIG. 6 is a diagram explaining voltages of a reading operation accordingto the comparative example.

FIG. 7 is a diagram explaining threshold voltage distributions andreading voltages of a reading operation according to a first embodiment.

FIG. 8 is a flowchart showing a data reading process according to thefirst embodiment.

FIG. 9 is a diagram explaining voltages of a reading operation accordingto the first embodiment.

FIG. 10 is a diagram explaining voltages of a reading operationaccording to the first embodiment.

FIG. 11 is a flowchart showing a data reading process according to asecond embodiment.

FIG. 12 is a diagram explaining voltages of a reading operationaccording to the second embodiment.

FIG. 13 is a diagram explaining threshold voltage distributions andreading voltages of a reading operation according to a third embodiment.

FIG. 14 is a diagram explaining threshold voltage distributions andreading voltages of a reading operation according to the thirdembodiment.

FIG. 15 is a flowchart showing a data reading process according to thethird embodiment.

FIG. 16 is a diagram explaining voltages of a reading operationaccording to the third embodiment.

FIG. 17 is a diagram explaining voltages of a reading operationaccording to the third embodiment.

FIG. 18 is a flowchart showing a data reading process according to amodified example of the third embodiment.

FIG. 19 is a diagram explaining threshold voltage distributions andreading voltages of a reading operation according to a fourthembodiment.

FIG. 20 is a flowchart showing a data reading process according to thefourth embodiment.

FIG. 21 is a diagram explaining voltages of a reading operationaccording to the fourth embodiment.

FIG. 22 is a diagram explaining voltages of a reading operationaccording to the fourth embodiment.

FIG. 23 is a flowchart showing a data reading process according to afifth embodiment.

FIG. 24 is a diagram explaining voltages of a reading operationaccording to the fifth embodiment.

FIG. 25 is a diagram explaining voltages of a reading operationaccording to the fifth embodiment.

FIG. 26 is a flowchart showing a data reading process according to asixth embodiment.

FIG. 27 is a diagram explaining voltages of a reading operationaccording to the sixth embodiment.

FIG. 28 is a diagram explaining threshold voltage distributions andreading voltages of a reading operation according to a seventhembodiment.

FIG. 29 is a flowchart showing a data reading process according to theseventh embodiment.

FIG. 30 is a diagram explaining voltages of a reading operationaccording to the seventh embodiment.

FIG. 31 is a diagram explaining voltages of a reading operationaccording to the seventh embodiment.

DETAILED DESCRIPTION

A nonvolatile semiconductor memory device according to one embodiment ofthe present invention includes: a memory cell array including memorystrings each including a plurality of memory cells connected in series,a first select transistor connected to one end of each memory string, asecond select transistor connected to the other end of each memorystring, bit lines connected to the memory strings through the firstselect transistors, a source line connected to the memory stringsthrough the second select transistors, and word lines connected tocontrol gate electrodes of the memory cells; and a control circuitconfigured to execute a reading operation of applying a reading voltageto the control gate electrode of a selected memory cell in a memorystring to read data and applying a reading pass voltage to non-selectedword lines connected to non-selected memory cells in the memory stringto thereby determine whether or not the selected memory cell becomesconductive. The control circuit is configured to be capable of executinga first reading operation and a second reading operation. The firstreading operation is an operation of reading a threshold voltage set inthe selected memory cell by setting a voltage between the control gateelectrode and source of the selected memory cell to a first value. Thesecond reading operation is an operation of reading a threshold voltageset in the selected memory cell by setting a voltage between the controlgate electrode and source of the selected memory cell to a second valuelower than the first value. When executing the second reading operation,the control circuit sets the voltage between the control gate electrodeand source to the second value while keeping a voltage of the controlgate electrode of the selected memory cell to 0 or a positive value.

Next, a nonvolatile semiconductor memory device according to theembodiments will be explained with reference to the drawings.

First Embodiment

[Configuration]

FIG. 1 is a block diagram showing a configuration of a nonvolatilesemiconductor memory device according to a first embodiment. Thenonvolatile semiconductor memory device is a NAND type flash memoryemploying a four-value storing system. The nonvolatile semiconductormemory device includes a memory cell array 1 configured by disposingmemory cells MC for storing data in a matrix arrangement. The memorycell array 1 includes a plurality of bit lines BL, a plurality of wordlines WL, a source line SRC, and a plurality of memory cells MC. Thememory cells MC are electrically-rewritable, and disposed at theintersections of the bit lines BL and word lines WL in a matrixarrangement.

The memory cell array 1 is connected to a bit line control circuit 2configured to control the voltages of the bit lines BL and a word linecontrol circuit 6 configured to control the voltages of the word linesWL. That is, the bit line control circuit 2 reads data of a memory cellMC in the memory cell array 1 through a bit line BL. The bit linecontrol circuit 2 also writes data in a memory cell MC in the memorycell array 1 through a bit line BL.

The bit line control circuit 2 is connected to a column decoder 3, adata input/output buffer 4, and a data input/output terminal 5. Data ofa memory cell MC read out from the memory cell array 1 is output toexternal through the data input/output terminal 5. Write data sent fromexternal (i.e. a memory controller or a host) and input to the datainput/output terminal 5 is input to the bit line control circuit 2 bythe column decoder 3, and written into a designated memory cell MC.

The memory cell array 1, the bit line control circuit 2, the columndecoder 3, the data input/output buffer 4, and the word line controlcircuit 6 are connected to a control circuit 7. In accordance withcontrol signals input to a control signal input terminal 8 from a memorycontroller or a host, the control circuit 7 generates control signalsfor controlling the bit line control circuit 2, the column decoder 3,the data input/output buffer 4, and the word line control circuit 6. Thecontrol circuit 7 may include a counter 7 a for counting the number oftimes writing operations is executed and the number of times erasingoperations is executed, and a timer 7 b for counting the cumulative timespent on operations.

FIG. 2 is a circuit diagram showing a configuration of the memory cellarray 1 shown in FIG. 1. As shown in FIG. 2, the memory cell array 1 isconfigured by a plurality of blocks B. In the memory cell array 1, datais erased on a block B basis (a block erasing process).

As shown in FIG. 2, a block B is configured by a plurality of memoryunits MU. One memory unit MU is configured by a memory string MSincluding, for example, sixteen memory cells MC connected in series, andfirst and second select transistors S1 and S2 connected to both ends ofthe memory string MS. One end of the first select transistor S1 isconnected to the bit line BL, and one end of the second selecttransistor S2 is connected to the source line SRC. The control gateelectrodes of memory cells MC arranged in line in the Y direction areconnected commonly to any of the word lines WL1 to WL16. The controlgate electrodes of the first select transistors S1 arranged in line inthe Y direction are connected commonly to a select line SG1, and thecontrol gate electrodes of the second select transistors S2 arranged inline in the Y direction are connected commonly to a select line SG2. Anaggregate P of a plurality of memory cells MC connected to one word lineWL constitutes one page or a plurality of pages. Writing or reading ofdata is executed on the aggregate P basis.

[Data Storage]

Next, an outline of a data storing scheme of the nonvolatilesemiconductor memory device will be explained with reference to FIG. 3.In the nonvolatile semiconductor memory device according to the presentembodiment, the threshold voltage of the memory cells MC can have, forexample, four distributions.

FIG. 3 shows a relationship between two-bit four-value data (data “11”,“01”, “10”, and “00”) to be stored in the memory cells MC of thenonvolatile semiconductor memory device and the threshold voltagedistributions of the memory cells MC. In FIG. 3, reading voltages AR,BR, and CR are voltages to be applied to the control gate of a selectedmemory cell MC (or a selected word line WL) which is selected whenreading four value data. A reading pass voltage Vread is a voltagewhich, in a data reading operation, is applied to the control gates ofnon-selected memory cells MC (or non-selected word lines WL) in a memorystring MS for making the non-selected memory cells MC conductiveregardless of the data retained therein. A voltage Vev is an erasingverify voltage which, when erasing data from the memory cells MC, isapplied to the memory cells MC in order to verify whether or not theerasing is completed.

The threshold voltage distribution E of the memory cells MC after blockerasing is assigned data “11”. Memory cells MC showing data “01”, “10”,and “00” representing written states are assigned threshold voltagedistributions A, B, and C, respectively. The threshold voltagedistribution A of the data “01” has the lowest voltage, the thresholdvoltage distribution C of the data “00” has the highest voltage, and thethreshold voltage distribution B of the data “10” has an intermediatevoltage between the threshold voltage distributions A and C. As shown inFIG. 3, two-bit data stored in one memory cell MC is composed of lowerpage data and upper page data. When data is represented as “*@”, “*”represents upper page data and “@” represents lower page data.

[Reading Operation]

Before the first embodiment will be explained, a reading operation of anonvolatile semiconductor memory device according to a comparativeexample will be explained. Normally, when executing a data readingoperation, any of the reading voltages AR, BR, and CR is supplied to theword line WL (selected word line WL1) connected to a selected memorycell MC in a memory unit MU. The reading pass voltage Vread is appliedto the word lines WL (non-selected word lines WL0, WL2, WL3, . . . )connected to non-selected memory cells MC. The bit line control circuit2 determines data by detecting whether or not a current flows throughthe memory unit MU at this time.

As described above, when data written in a memory cell MC remainsun-accessed for a long time, electrons are discharged from the chargeaccumulation layer of the memory cell MC to change the threshold voltageof the memory cell MC to lower (deterioration of data retention).Influence of this deterioration of data retention will be explained withreference to a comparative example shown in FIG. 4.

FIG. 4 is a diagram explaining the threshold voltage distributions andreading voltages of a reading operation according to a comparativeexample. As shown in FIG. 4, due to deterioration of data retention, thethreshold voltage distributions (solid lines) of the memory cells MCchange to become lower than the threshold voltage distributions (brokenlines) which were given when writing. At this time, the lower limitvalues of the threshold voltage distributions A, B, and C might becomelower than the reading voltages AR, BR, and CR, respectively. In thiscase, a reading operation for reading the threshold voltages of thememory cells MC may fail.

One example of a scheme for a reading operation for dealing withdeterioration of data retention, and problems involved with this schemewill be explained with reference to the comparative example shown inFIG. 4 to FIG. 6. In order to prevent erroneous reading due todeterioration of data retention, the semiconductor memory deviceaccording to the comparative example executes a reading operation(second reading operation) which uses reading voltages different fromthe reading voltages used in a normal reading operation (first readingoperation). That is, when data cannot be read out correctly by thenormal reading operation (first reading operation) resulting in manyreading errors, a data reading operation (second reading operation) isagain executed by changing the voltage to be applied to the selectedword line WL from the reading voltages AR, BR, or CR of the normalreading operation to reading voltages ARs (<AR), BRs (<BR), or CRs(<CR), as shown in FIG. 4. The reading voltages ARs, BRs, or CRs are setto values lower than the lower limit values of the threshold voltagedistributions broadened at the lower side (shown by solid lines in FIG.4). Also, the reading voltages ARs, BRs, and CRs are set between theupper limit and the lower limit of the threshold voltage distributionsbroadened at the lower side. By using such reading voltages ARs, BRs,and CRs, it is possible to execute a data reading operation on thememory cell MC correctly.

A reading operation to be executed by changing the reading voltage to beapplied to the selected word line WL from the original voltages AR, BR,or CR to the voltages ARs, BRs, or CRs to lower the voltage between thecontrol gate electrode and source of the selected memory cell MC will behereinafter referred to as “shift reading operation”. Here, the voltagebetween the control gate electrode and source can be regarded simply asa voltage between the control gate electrode and the source line or avoltage between the control gate electrode and the bit line. Also,“setting the voltage between the control gate electrode and source” canbe said to be equivalent to “controlling the voltage of the control gateelectrode and the voltage of the source line or the bit line”.

The process of a data reading operation according to the comparativeexample will be explained with reference to FIG. 5 and FIG. 6. First,when a reading signal is input to the control circuit 7, a data readingprocess is started (step S1). Next, a normal reading operation (firstreading operation) using the reading voltages AR, BR, and CR is executedby the control circuit 7 (step S2). After the normal reading operation,ECC check is executed (step S3). When there are few reading errors anderror correction is possible, the data reading process is finished, bydetermining that data reading is executed correctly (step S7).

However, when there are many reading errors and error correction isimpossible, the process goes to a second reading operation, bydetermining that deterioration of data retention occurs (step S4). Inthe second reading operation, a shift reading operation is executed bychanging the voltage applied to the selected word line WL from thenormal reading voltages AR, BR, CR to the voltages ARs, BRs, and CRs.After the second reading operation, ECC check is executed (step S5).When there are few reading errors and error correction is possible, thereading process is finished, by determining that data reading isexecuted correctly (step S7). However, when even the second readingoperation results in many reading errors and error correction isimpossible, the reading process is finished, by determining that datareading fails (step S6 and step S7).

FIG. 6 is a diagram explaining the voltage to be applied to a selectedword line WL in a reading operation. In a normal reading operation(first reading operation), the reading voltages AR, BR, or CR is appliedas a reading voltage to be applied to the selected word line WL.

In a second reading operation, a shift reading operation is executed bychanging the reading voltage to the voltages ARs (<AR), BRs (<BR), orCRs (<CR). Here, when the threshold voltage distribution E is adistribution having negative threshold voltage values and the thresholdvoltage distributions A, B, and C are distributions having positivethreshold voltage values, there can be a case when the reading voltageARs in a shift reading operation has to be set to a negative voltagevalue, depending on the value of the original reading voltage AR.However, in order to set a negative reading voltage, it is necessary toprovide a voltage generating circuit for generating a negative voltage,which leads to problems that the area of the peripheral circuits isincreased, that the power consumption of the peripheral circuits isincreased, or the like.

[Reading Operation According to First Embodiment]

In consideration of the problems of the shift reading operationaccording to the comparative example described above, the nonvolatilesemiconductor memory device according to the present embodiment executesa reading operation explained below. The reading operation according tothe present embodiment will now be explained with reference to FIG. 7 toFIG. 10.

As shown in FIG. 7, in a second reading operation after a first readingoperation fails, a shift reading operation is executed by changing thereading voltage to be applied to the selected word line WL from thenormal reading voltages BR or CR to the voltages BRs or CRs, which isthe same as in the comparative example. Since the voltages BRs and CRsare both positive voltages, there is no problem with executing the shiftreading operation.

On the other hand, in a second reading operation for determining whetherthe threshold voltage of a memory cell is included in the thresholdvoltage distribution E, or in any of the threshold voltage distributionsA to C, no shift reading operation is executed. That is, in the secondreading operation, the reading voltage to be applied to the word line WLis not changed from the normal reading voltage AR to the voltage ARs,but the reading voltage AR is used. Instead, the voltage of the sourceline SRC is raised from 0 V to a voltage Vsrc (>0). This is for loweringthe voltage between the control gate electrode and source of theselected memory cell MC. In terms of the voltage between the controlgate electrode and source, raising the voltage of the source line SRCfrom 0 V to Vsrc is equivalent to lowering the voltage of the controlgate electrode (the selected word line WL) from AR to ARs. Therefore,raising the voltage of the source line SRC from 0 V to Vsrc results inthe same state as resulting from executing shift reading by lowering thevoltage AR (see FIG. 7).

The process of the data reading operation according to the presentembodiment will be explained with reference to FIG. 8. Step S11 to stepS13 (first reading operation, or the like.) of FIG. 8 are the same ascorresponding step S1 to step S3 according to the comparative exampleshown in FIG. 5. In a second reading operation after the first readingoperation fails, the same shift reading operation as in the comparativeexample is executed by changing the voltage to be applied to theselected word line WL from the reading voltages BR or CR to the voltagesBRs or CRs (step S14). However, in the second reading operationaccording to the present embodiment, reading operation is executed bynot changing the reading voltage AR to the voltage ARs, but instead byraising the voltage of the source line SRC from 0 V to the voltage Vsrc(step S14). Note that the value of a well voltage Vwell used in step S14is equal to the voltage Vsrc.

After the second reading operation, ECC check is executed (step S15).When there are few reading errors and error correction is possible, thereading process is finished by determining that data reading is executedcorrectly (step S17). However, when even the second reading operationresults in many reading errors and error correction is impossible, thereading process is finished by determining that data reading fails (stepS16 and step S17).

FIG. 9 is a diagram explaining the voltages to be applied in a readingoperation. In a normal reading operation (first reading operation), thereading voltages AR, BR, or CR is applied to the selected word line WL.

On the other hand, in a second reading operation to be executed whencorrect reading is impossible by the first reading operation, thevoltage between the control gate electrode and source of the selectedmemory cell MC is set to a value lower than that in the first readingoperation. In this way, the threshold voltage set in the selected memorycell MC is read out. In this case, the reading voltages BR and CR arechanged to the voltages BRs and CRs respectively, and a shift readingoperation is executed.

Here, the reading voltage AR is not changed to the voltage ARs. That is,in a reading operation for determining whether the threshold voltage isin the distribution E or in any of the distributions A, B, and C, noshift reading operation is executed. Instead, the voltage of the sourceline SRC is raised from 0 V to the voltage Vsrc, thereby the voltagebetween the control gate electrode and source is set to a value lowerthan the voltage in the first reading operation (step S12). In thiscase, even in the second reading operation, the voltage of the controlgate electrode of the selected memory cell MC is kept at 0 or a positivevalue.

FIG. 10 is a diagram showing states when the voltage AR is applied tothe control gate electrode of the selected memory cell MC in a datareading operation. FIG. 10 shows a cross-sectional view of the selectedmemory cell MC. In a first reading operation, the voltage AR is appliedto the selected word line WL. Further, for example, 0 V is supplied as asource line voltage to the source of the selected memory cell MC, andthe voltage of the bit line BL (for example, 0.5 V) is supplied to thedrain of the selected memory cell MC. Based on whether the selectedmemory cell MC becomes conductive at this time, the threshold of theselected memory cell MC is detected. On the other hand, in a secondreading operation, the voltage AR of the selected memory cell MC is notchanged, but is kept at, for example, 0 V. Then, the source is raised tothe voltage Vsrc (for example, 1 V), thereby the voltage between thecontrol gate electrode and source is changed from what it is in thefirst reading operation. This voltage application state is equivalent toa state when reading operation is executed by applying a negativevoltage to the selected word line WL.

[Effect]

In the reading operation according to the present embodiment, whendeterioration of data retention occurs and the threshold voltagedistributions broaden at the lower side, a shift reading operation isexecuted by changing the values of the reading voltages BR and CR to thevoltages BRs and CRs. On the other hand, the value of the readingvoltage AR is not changed (or not set to a negative value), but readingoperation is executed by raising the voltage of the source line SRC from0 V to the voltage Vsrc. By employing such a data reading scheme in thesecond reading operation executed after deterioration of data retentionoccurs, it is possible to execute a data reading operation on the memorycell MC correctly. In addition, since the voltage to be applied to theselected memory cell MC need not to be a negative value, voltage controlin the reading operation becomes easier.

Furthermore, a shift reading operation is executed for the thresholdvoltage distributions B and C which are less likely to become a negativethreshold voltage at their lower side due to deterioration of dataretention. Hence, a positive voltage is not applied to the source lineSRC in a reading operation for the threshold voltage distributions B andC, which leads to less power consumption and a higher operation speed.

Second Embodiment

Next, a nonvolatile semiconductor memory device according to a secondembodiment will be explained with reference to FIG. 11 and FIG. 12. Thewhole configuration of the nonvolatile semiconductor memory deviceaccording to the present embodiment is the same as the first embodiment,and hence a detailed explanation thereof will not be provided here.Further, any portions that are configured the same as the firstembodiment will be denoted by the same reference numerals, and aredundant explanation thereof will not be provided.

In the second reading operation according to the first embodiment, areading operation is executed by changing the voltage to be applied tothe selected word line WL from the reading voltages BR or CR to thevoltages BRs or CRs to execute a shift reading operation, and by notchanging the reading voltage AR to the voltage ARs but instead raisingthe voltage of the source line SRC from 0 V to the voltage Vsrc. Incontrast, in the second reading operation according to the presentembodiment, a reading operation is executed by keeping all of thereading voltages AR, BR, and CR unchanged and raising the voltage of thesource line SRC from 0 V to the voltage Vsrc regardless of the voltageto be applied to the selected word line WL. Explanation will now begiven with reference to FIG. 11 and FIG. 12.

The process of the data reading operation according to the presentembodiment will be explained with reference to FIG. 11. Step S21 to stepS23 of FIG. 11 are the same as corresponding step S11 to step S13according to the first embodiment shown in FIG. 8. In the second readingoperation according to the present embodiment, reading operation isexecuted by using the reading voltages AR, BR, and CR as they arewithout changing them as in the first reading operation, but instead byraising the voltage of the source line SRC from 0 V to the voltage Vsrcregardless of which of the voltages AR, BR, and CR is to be applied tothe selected word line WL (step S24). Note that the voltage of a wellvoltage Vwell used in step S24 is equal to the voltage Vsrc.

After the second reading operation, ECC check is executed (step S25).When there are few reading errors and error correction is possible, thereading process is finished by determining that data reading is executedcorrectly (step S27). However, when even the second reading operationresults in many reading errors and error correction is impossible, thereading process is finished by determining that data reading fails (stepS26 and step S27).

FIG. 12 is a diagram explaining the voltages to be applied in a readingoperation. In the normal reading operation (first reading operation),the reading voltages AR, BR, or CR is applied to the selected word lineWL. Also according to the present embodiment, in the second readingoperation, the voltage between the control gate electrode and source ofthe selected memory cell MC is set to a value lower than that in thefirst reading operation to determine the threshold voltage of theselected memory cell MC. However, the voltage to be applied to theselected word line WL is kept at the reading voltages AR, BR, or CR(i.e., no shift reading operation is executed). Instead, the voltage ofthe source line SRC is raised from 0 V to the voltage Vsrc, and therebythe voltage between the control gate electrode and source of the memorycell is set to a value lower than the value in the first readingoperation. Also according to the present embodiment, even in the secondreading operation, the voltage of the control gate electrode of theselected memory cell MC is kept at 0 or a positive value.

[Effect]

In the reading operation according to the present embodiment, whendeterioration of data retention occurs and the threshold voltagedistributions broaden at the lower side, reading operation is executedby not changing the voltage to be applied to the selected word line WLfrom the reading voltages AR, BR, or CR but instead by raising thevoltage of the source line SRC from 0 V to the voltage Vsrc. Since thereading voltages BRs and CRs are not used in the present embodiment, thenumber of reading voltages to be set does not increase, and a simplifiedcontrol on the reading operation can be realized.

As described above, in terms of the voltage between the control gateelectrode and source, raising the voltage of the source line SRC from 0V to Vsrc is equivalent to lowering the voltage of the control gateelectrode (the selected word line WL). Hence, raising the source lineSRC to the voltage Vsrc results in the same state as resulting fromlowering the voltages AR, BR, or CR. It is possible to execute a datareading operation on the memory cell MC correctly, by employing such adata reading scheme in the second reading operation which is executedafter it is determined that correct reading is impossible by the firstreading operation due to deterioration of data retention. Since thevoltage to be applied to the selected memory cell MC need not to be anegative value, voltage control in the reading operation becomes easier.Furthermore, it becomes possible for the second reading operation to beexecuted by keeping the voltage to be applied to the source line SRC atthe voltage Vsrc and not changing it, which makes it possible to reducethe time necessary for reading.

Third Embodiment

Next, a nonvolatile semiconductor memory device according to a thirdembodiment will be explained with reference to FIG. 13 to FIG. 17. Thewhole configuration of the nonvolatile semiconductor memory deviceaccording to the present embodiment is the same as the first embodiment,and hence a detailed explanation thereof will not be provided. Anyportions that are configured the same as the first embodiment will bedenoted by the same reference numerals, and a redundant explanationthereof will not be provided.

In the first and second embodiments, it is explained that the voltage tobe applied to the source line SRC in the second reading operation is onetype of voltage, that is, the voltage Vsrc. In contrast, according tothe present embodiment, the voltage to be applied to the source line SRCin the second reading operation can be set to a plurality of positivevalues, for example, two positive values. Explanation will now be givenwith reference to FIG. 13 and FIG. 14.

When the gate insulating film deteriorates due to repetitive writing anderasing operations to the memory cell MC or when a long time passesafter data is written in the memory cell MC, more of the electronsretained in the charge accumulation layer are discharged to lower thethreshold voltage of the memory cell MC greatly. In this case,broadening of the threshold voltage distributions due to deteriorationof data retention is larger as shown in FIG. 13. Hence, there is apossibility that the threshold voltage distributions A, B, or C cannotbe read out correctly even by applying the voltage Vsrc to the sourceline SRC in the second reading operation as explained in the first andsecond embodiments, because applying the voltage Vsrc might not beenough for the lower limit value of the threshold voltage distributionsA, B, or C to be read out.

In the reading operation according to the third embodiment, when thegate insulation film deteriorates due to repetitive writing and erasingoperations or when a long time passes after data is written, the voltageto be applied to the source line SRC in the second reading operation ischanged from the voltage Vsrc to a still higher voltage Vsrc′ (>Vsrc).By adjusting the value of the voltage to be applied to the source lineSRC appropriately, it is possible to manage even when data retentiondeteriorates greatly as shown in FIG. 14. In addition, a shift readingoperation may be executed by changing the reading voltages BR or CR tovoltages BRs′ or CRs′ (BRs′<BRs, CRs′<CRs). Determination of whether ornot to change the voltage of the source line SRC to the voltage Vsrc′can be made based on the counted value of the counter 7 a (FIG. 1) forcounting the number of times data writing/erasing operations areexecuted in the memory cell MC, the counted value of the timer 7 b forcounting the cumulative time spent on operations on the memory cell MC,or the like. When executing the second reading operation after thecounted value of the counter 7 a or the timer 7 b exceeds a referencevalue, the voltage of the source line SRC may be changed from thevoltage Vsrc to the voltage Vsrc′.

The process of the data reading operation according to the thirdembodiment will be explained with reference to FIG. 15. Step S31 to stepS33 of FIG. 15 are the same as corresponding step S11 to step S13 of thefirst embodiment shown in FIG. 8. In the present embodiment, when it isdetermined by ECC check executed after the first reading operation thatdeterioration of data retention occurs, the counted value of the counter7 a or the timer 7 b is read out (step S34). When the counted value ofthe counter 7 a or the timer 7 b is equal to or smaller than thereference value, the reading operation is executed by selecting thevoltage Vsrc as the voltage of the source line SRC, by determining thatbroadening of the threshold voltage distributions due to deteriorationof data retention is small (step S35).

On the other hand, when the counted value of the counter 7 a or thetimer 7 b is larger than the reference value, the reading operation isexecuted by selecting the voltage Vsrc′ as the voltage of the sourceline SRC, by determining that broadening of the threshold voltagedistributions due to deterioration of data retention is large (stepS36). Note that the value of a well voltage Vwell used in step S35 orS36 is equal to the voltage Vsrc.

After the second reading operation, ECC check is executed (step S37).When there are few reading errors and error correction is possible, thereading process is finished by determining that data reading is executedcorrectly (step S39). However, when even the second reading operationresults in many reading errors and error correction is impossible, thereading process is finished by determining that data reading fails (stepS38 and step S39).

In step S36 of FIG. 15, the second reading operation may be executed byexecuting a shift reading operation by changing the reading voltages BRand CR to the voltages BRs′ and CRs′ (BRs′<BRs, CRs′<CRs), as shown inFIG. 16. Only when the reading voltage AR is to be applied to theselected word line WL, the voltage of the source line SRC rises to thevoltage Vsrc′ instead of changing the voltage value of the readingvoltage AR. Alternatively, as shown in FIG. 17, the second readingoperation may be executed by not changing the voltage values of thereading voltages AR, BR, and CR but raising the voltage to be applied tothe source line SRC to the voltage Vsrc′ regardless of which of thevoltages AR, BR, and CR is to be applied to the selected word line WL.Also according to the present embodiment, the voltage of the controlgate electrode of the selected memory cell MC is kept at 0 or a positivevalue in the second reading operation.

[Effect]

In the reading operation according to the present embodiment, when thethreshold voltage distributions broaden, it is possible to select, basedon the width of the broadening, which of the voltages Vsrc and Vsrc′ touse as the voltage to be applied to the source line SRC for executingthe reading operation. It is possible to execute a data readingoperation on the memory cell MC correctly by employing such a datareading scheme in the second reading operation which is executed afterdeterioration of data retention occurs. Since the voltage to be appliedto the selected memory cell MC need not to be a negative value, voltagecontrol in the reading operation becomes easier.

[Modified Example of Third Embodiment]

Next, a nonvolatile semiconductor memory device according to a modifiedexample of the third embodiment will be explained with reference to FIG.18. In the third embodiment described above, determination of whether ornot to change the value of the voltage to be applied to the source lineSRC in the second reading operation to the voltage Vsrc′ is made basedon the counted values of the counter 7 a or the timer 7 b, or the like.Here, determination of whether or not to change the value of the voltageto be applied to the source line SRC to the voltage Vsrc′ may be madebased on the result of a reading operation which is executed by settingthe value of the voltage to be applied to the source line SRC to thevoltage Vsrc.

The process of the data reading operation according to the modifiedexample of the third embodiment will be explained with reference to FIG.18. Step S31′ to step S33′ of FIG. 18 are the same as corresponding stepS31 to step S33 of the third embodiment shown in FIG. 15. In the presentembodiment, when it is determined by ECC check executed after the firstreading operation that deterioration of data retention occurs, thesecond reading operation is executed by selecting the voltage Vsrc asthe voltage of the source line SRC (step S34′).

After the second reading operation, ECC check is executed (step S35′).When it is determined by this ECC check that deterioration of dataretention occurs, a third reading operation is executed by selecting thevoltage Vsrc′ as the voltage of the source line SRC, by determining thatbroadening of the threshold voltage distributions due to deteriorationof data retention is large (step S36′). Note that the value of a wellvoltage Vwell used in steps S34′ and S36′ is equal to the voltage Vsrc.

After the third reading operation, ECC check is executed (step S37′).When there are few reading errors and error correction is possible, thereading process is finished by determining that data reading is executedcorrectly (step S39′). However, when even the third reading operationresults in many reading errors and error correction is impossible, thereading process is finished by determining that data reading fails (stepS38′ and step S39′).

[Effect]

In the reading operation according to the present modified example, itis possible to select whether to execute reading operation by using thevoltage Vsrc′, based on the result of the reading operation executed byapplying the voltage Vsrc to the source line SRC. By employing such adata reading scheme in the second reading operation which is executedafter deterioration of data retention occurs, it is possible to executea data reading operation on the memory cell MC correctly. Furthermore,the reading operation according to the present modified example allowsdata reading to be executed correctly even if the counter 7 a and thetimer 7 b are not provided.

Fourth Embodiment

Next, a nonvolatile semiconductor memory device according to a fourthembodiment will be explained with reference to FIG. to FIG. 22. Thewhole configuration of the nonvolatile semiconductor memory deviceaccording to the present embodiment is the same as the first embodiment,and hence a detailed explanation thereof will not be provided. Anyportions that are configured the same as the first embodiment will bedenoted by the same reference numerals, and a redundant explanationthereof will not be provided.

In the first to third embodiment described above, it is explained thatthe reading pass voltage Vread to be applied to the non-selected memorycells MC in the reading operation has a fixed value. In contrast,according to the fourth embodiment, the value of the reading passvoltage Vread used in the reading operation can be changed in accordancewith the voltage to be applied to the selected word line WL. Explanationwill now be given with reference to FIG. 19.

When the reading pass voltage Vread is applied to non-selected memorycells MC adjoining the selected memory cell MC in the reading operationof the selected memory cell MC (here after may be called “adjoiningnon-selected memory cells MC”), the threshold voltage of the selectedmemory cell MC seems to become lower due to the influence of the readingpass voltage Vread. When the reading pass voltage Vread is changed to avoltage Vread′ (Vread>Vread′), the threshold voltage of the selectedmemory cell MC seems to become higher, that is, an effect that thereading voltage AR becomes lower is achieved. Not only the voltage Vsrcof the source line SRC described above but also the influence caused byapplying the reading pass voltage Vread′ to the adjoining non-selectedmemory cells MC enable the value of the reading voltage AR to beadjusted so that data reading may be executed correctly. Note that thevoltage Vread′ may be applied to the control gate electrodes of only theadjoining non-selected memory cells MC.

The process of the data reading operation according to the presentembodiment will now be explained with reference to FIG. 20. Step S41 tostep S43 of FIG. 20 are the same as corresponding step S11 to step S13of the first embodiment shown in FIG. 8. In the second reading operationaccording to the present embodiment, reading operation is executed byraising the voltage of the source line SRC from 0 V to Vsrc and changingthe reading pass voltage to be applied to the non-selected memory cellsMC to the voltage Vread′ (step S44). The value of a well voltage Vwellused in step S44 is equal to the voltage Vsrc. After the second readingoperation, ECC check is executed (step S45). When there are few readingerrors and error correction is possible, the reading process is finishedby determining that data reading is executed correctly (step S47).However, when even the second reading operation results in many readingerrors and error correction is impossible, the reading process isfinished by determining that data reading fails (step S46 and step S47).

FIG. 21 and FIG. 22 are diagrams explaining the voltages to be appliedin a reading operation. In the normal reading operation (first readingoperation), the reading voltages AR, BR, or CR is applied to theselected word line WL. Also according to the present embodiment, in thesecond reading operation, the voltage between the control gate electrodeand source of the selected memory cell MC is set to a value lower thanthat in the first reading operation to determine the threshold voltageof the selected memory cell MC.

In step S44 of FIG. 20, the second reading operation may be executed byexecuting a shift reading operation by changing the reading voltages BRand CR to the voltages BRs and CRs without changing the reading voltageVread, as shown in FIG. 21. Only when the voltage AR is to be applied tothe selected word line WL, the voltage of the source line SRC rises tothe voltage Vsrc and the reading pass voltage Vread′ is used to executethe second reading operation instead of changing the voltage value ofthe reading voltage AR. Alternatively, the second reading operation maybe executed by not changing the voltage values of the reading voltagesAR, BR, and CR but raising the voltage to be applied to the source lineSRC to the voltage Vsrc and using the reading pass voltage Vread′regardless of which of the voltages AR, BR, and CR is to be applied tothe selected word line WL, as shown in FIG. 22. Also according to thepresent embodiment, the voltage of the control gate electrode of theselected memory cell MC is kept at 0 or a positive value in the secondreading operation.

[Effect]

In the reading operation according to the present embodiment, it ispossible to select which of the reading pass voltages Vread and Vread′to use for executing reading operation. The values of the readingvoltages can be adjusted based also on the level of the values of thereading pass voltages Vread and Vread′. By employing such a data readingscheme in the second reading operation, it is possible to execute a datareading operation on the memory cell MC correctly. Since the voltage tobe applied to the selected memory cell MC need not to be a negativevalue, voltage control in the reading operation becomes easier.

Furthermore, since the voltage of the control gate electrode of thenon-selected memory cells MC can be lowered, it is possible to preventfalse writing due to so-called read disturb.

Fifth Embodiment

Next, a nonvolatile semiconductor memory device according to a fifthembodiment will be explained with reference to FIG. 23 to FIG. 25. Thewhole configuration of the nonvolatile semiconductor memory deviceaccording to the present embodiment is the same as the first embodiment,and a detailed explanation thereof will not be provided. Any portionsthat are configured the same as the first embodiment will be denoted bythe same reference numerals, and a redundant explanation thereof willnot be provided.

According to the present embodiment, the value of a well voltage Vwellused in the reading operation to be applied to the memory cell MC can bechanged in accordance with the voltage to be applied to the selectedword line WL. Normally, the well voltage Vwell is set to a voltage equalto the voltage of the source line SRC. However, raising the voltage ofthe source line SRC from 0 V to the voltage Vsrc and lowering the valueof the well voltage Vwell in the reading operation results in the samestate as a state in which a back-gate bias is applied. In this case, thethreshold voltage of the selected memory cell MC seems to become higher.Not only changing the voltage Vsrc of the source line SRC describedabove but also changing of the well voltage Vwell enable the value ofthe reading voltage to be adjusted so that data reading may be executedcorrectly.

The process of the data reading operation according to the presentembodiment will now be explained with reference to FIG. 23. Step S51 tostep S53 of FIG. 23 are the same as corresponding step S11 to step S13of the first embodiment shown in FIG. 8. In the second reading operationaccording to the present embodiment, reading operation is executed byraising the voltage of the source line SRC from 0 V to the voltage Vsrcand changing the well voltage Vwell (step S54). After the second readingoperation, ECC check is executed (step S55). When there are few readingerrors and error correction is possible, the reading process is finishedby determining that data reading is executed correctly (step S57).However, when even the second reading operation results in many readingerrors and error correction is impossible, the reading process isfinished by determining that data reading fails (step S56 and step S57).

FIG. 24 and FIG. 25 are diagrams explaining the voltages to be appliedin the reading operation. In the normal reading operation (first readingoperation), the reading voltages AR, BR, or CR is applied to theselected word line WL. Also according to the present embodiment, in thesecond reading operation, the voltage between the control gate electrodeand source of the selected memory cell MC is set to a value lower thanthat in the first reading operation to determine the threshold voltageof the selected memory cell.

In step S54 of FIG. 23, the second reading operation may be executed byexecuting a shift reading operation by changing the reading voltages BRand CR to the voltages BRs and CRs, as shown in FIG. 24. Only when thevoltage AR is to be applied to the selected word line WL, the voltage ofthe source line SRC rises to the voltage Vsrc instead of changing thevoltage value of the reading voltage AR. In this case, when the wellvoltage Vwell is lowered to below the source line voltage Vsrc, thethreshold voltage of the selected memory cell MC seems to become higher.Alternatively, the second reading operation may be executed by notchanging the voltage values of the reading voltages AR, BR, and CR butraising the voltage to be applied to the source line SRC to the voltageVsrc and lowering the well voltage Vwell to below the voltage Vsrcregardless of which of the voltages AR, BR, and CR is to be applied tothe selected word line WL, as shown in FIG. 25. Also according to thepresent embodiment, the voltage of the control gate electrode of theselected memory cell MC is kept at 0 or a positive value in the secondreading operation.

[Effect]

In the reading operation according to the present embodiment, readingoperation is executed by raising the voltage of the source line SRC tothe voltage Vsrc and lowering the well voltage Vwell. Lowering the wellvoltage Vwell makes the threshold voltage of the selected memory cellseem to become higher. By employing such a data reading scheme in thesecond reading operation, it is possible to execute the data readingoperation on the memory cell MC correctly. Since the voltage to beapplied to the selected memory cell MC need not to be a negative value,voltage control in the reading operation becomes easier.

Furthermore, lowering the well voltage Vwell enables the time requiredfor charging the well to be reduced. This leads to an improved operationspeed.

Sixth Embodiment

Next, a nonvolatile semiconductor memory device according to a sixthembodiment will be explained with reference to FIG. 26 and FIG. 27. Thewhole configuration of the nonvolatile semiconductor memory deviceaccording to the present embodiment is the same as the first embodiment,and hence a detailed explanation thereof will not be provided. Anyportions that are configured the same as the first embodiment will bedenoted by the same reference numerals, and a redundant explanationthereof will not be provided.

In the first to fifth embodiments described above, the voltage to beapplied to the source line SRC in the first reading operation is set to0 V, and the voltage to be applied to the source line SRC in the secondreading operation is set to the voltage Vsrc or the voltage Vsrc′(Vsrc′>Vsrc). Here, the first reading operation is not limited thenormal reading operation in which the voltage of the source line SRC isset to 0V. In the first reading operation, it is only necessary that thevoltage between the control gate electrode and source of the selectedmemory cell MC be set to a value higher than that in the second readingoperation. Hence, according to the sixth embodiment, the readingoperation is executed by setting the voltage to be applied to the sourceline SRC in the first reading operation to the voltage Vsrc and thevoltage to be applied to the source line SRC in the second readingoperation to the voltage Vsrc′.

The process of the data reading operation according to the presentembodiment will be explained with reference to FIG. 26. First, when areading signal sent from external is input to the control circuit 7, thedata reading process is started (step S61). Next, the first readingoperation is executed by applying the voltage Vsrc to the source lineSRC (step S62). The value of the well voltage Vwell used in step S62 isequal to the voltage Vsrc. After the first reading operation, ECC checkis executed (step S63). When there are few reading errors and errorcorrection is possible, the data reading process is finished bydetermining that data reading is executed correctly (step S67). In thesecond reading operation executed after the first reading operationfails, reading operation is executed by raising the voltage of thesource line SRC from the voltage Vsrc to the voltage Vsrc′ (step S64).The value of the well voltage Vwell used in step S64 is equal to thevoltage Vsrc′.

After the second reading operation, ECC check is executed (step S65).When there are few reading errors and error correction is possible, thereading process is finished by determining that data reading is executedcorrectly (step S67). However, when even the second reading operationresults in many reading errors and error correction is impossible, thereading process is finished by determining that data reading fails (stepS66 and step S67).

FIG. 27 is a diagram explaining the voltages to be applied in thereading operation. In the first reading operation, the reading voltagesAR, BR, or CR are applied to the selected word line WL, and the voltageVsrc is applied to the source line SRC. In the second reading operation,the voltage of the source line SRC is raised from the voltage Vsrc tothe voltage Vsrc′ to execute reading operation. According to the presentembodiment, the voltage of the control gate electrode of the selectedmemory cell MC is kept at 0 or a positive value in the first and secondreading operations.

[Effect]

In the reading operation according to the present embodiment, thevoltage Vsrc or the voltage Vsrc′ is applied as the voltage of thesource line SRC for executing reading operation. By employing such adata reading scheme, it is possible to execute reading without settingthe reading voltage AR to a negative value, even when the thresholdvoltage distribution A has negative voltage at the lower side under acondition that no deterioration of data retention occurs. Further, whendeterioration of data retention occurs, the voltage Vsrc′ (>Vsrc) isused as the voltage of the source line SRC, allowing the data readingoperation on the memory cell MC to be executed correctly. Since thevoltage to be applied to the selected memory cell MC need not to be anegative value, voltage control in the reading operation becomes easier.

Furthermore, when deterioration of data retention occurs, the readingoperation may be skipped, and the normal reading operation using thevoltage Vsrc may be performed. Then, it is possible to reduce the timeof the data reading operation.

Seventh Embodiment

Next, a nonvolatile semiconductor memory device according to a seventhembodiment will be explained with reference to FIG. to FIG. 31. Thewhole configuration of the nonvolatile semiconductor memory deviceaccording to the present embodiment is the same as the first embodiment,and hence a detailed explanation thereof will not be provided. Anyportions that are configured the same as the first embodiment will bedenoted by the same reference numerals, and a redundant explanationthereof will not be provided.

In the fourth embodiment described above, it is explained that theoperation of changing the reading pass voltage Vread to the voltageVread′ is executed together with the operation of raising the voltage ofthe source line SRC to the voltage Vsrc. In contrast, according to thepresent embodiment, the value of the reading pass voltage Vread ischanged without changing the voltage to be applied to the source lineSRC. Explanation will now be given with reference to FIG. 28.

In the reading operation on the selected memory cell MC, when thereading pass voltage Vread is applied to the adjoining non-selectedmemory cells MC, the threshold voltage of the selected memory cell MCseems to become lower due to the influence of the reading pass voltageVread. When the reading pass voltage Vread is changed to the voltageVread′ (Vread>Vread′), the threshold voltage of the selected memory cellMC seems to become higher, that is, an effect that the reading voltageAR becomes lower is achieved. Only the influence caused by applying thereading pass voltage Vread′ to the adjoining non-selected memory cellsMC without any other effect also enables the value of the readingvoltage AR to be adjusted so that data reading for the threshold voltagedistribution A can be executed correctly.

The process of the data reading operation according to the presentembodiment will be explained with reference to FIG. 29. Step S71 to stepS73 of FIG. 29 are the same as corresponding step S41 to step S43 of thefourth embodiment shown in FIG. 20. In the second reading operationaccording to the present embodiment, reading operation is executed bychanging the reading pass voltage to be applied to the non-selectedmemory cells MC to the voltage Vread′ without raising the voltage of thesource line SRC from 0 V (step S74). The value of the well voltage Vwellused in step S74 is equal to the voltage Vsrc. After the second readingoperation, ECC check is executed (step S75). When there are few readingerrors and error correction is possible, the reading process is finishedby determining that data reading is executed correctly (step S77).However, when even the second reading operation results in many readingerrors and error correction is impossible, the reading process isfinished by determining that data reading fails (step S76 and step S77).

FIG. 30 and FIG. 31 are diagrams explaining the voltages to be appliedin the reading operation. In the normal reading operation (first readingoperation), the reading voltages AR, BR, or CR are applied to theselected word line WL. In step S74 of FIG. 29, the second readingoperation may be executed by executing a shift reading operation bychanging the voltages BR and CR to the voltages BRs and CRs withoutchanging the reading pass voltage Vread, as shown in FIG. 30. Only whenthe reading voltage AR is to be applied to the selected word line WL,the reading pass voltage Vread′ is used instead of changing the voltagevalue of the reading voltage AR. Alternatively, the second readingoperation may be executed by not changing the voltage values of thereading voltages AR, BR, and CR but by using the reading pass voltageVread′ regardless of which of the voltages AR, BR, and CR is to beapplied to the selected word line WL, as shown in FIG. 31. Alsoaccording to the present embodiment, the voltage of the control gateelectrode of the selected memory cell MC is kept at 0 or a positivevalue in the second reading operation.

[Effect]

In the reading operation according to the present embodiment, it ispossible to select which of the reading pass voltages Vread and Vread′to use for executing reading operation. The value of the reading voltagecan be adjusted based also on the level of the values of the readingpass voltages Vread and Vread′. By employing such a data reading schemein the second reading operation, it is possible to execute the datareading operation on the memory cell MC correctly. Since the voltage tobe applied to the selected memory cell MC need not to be a negativevalue, voltage control in the reading operation becomes easier.

[Others]

While certain embodiments of the present invention have been described,these embodiments have been presented by way of example only, and arenot intended to limit the scope of the inventions. Indeed, the novelembodiments described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the embodiments described herein may be made without departingfrom the spirit of the inventions. The accompanying claims and theirequivalents are intended to cover such forms or modifications as wouldfall within the scope and spirit of the inventions. For example, theembodiments described above explain a nonvolatile semiconductor memorydevice of a four-value storing system (2 bits/cell). However, thepresent invention is not limited to this, but needless to say, can beapplied to a storing system for more bits, such as an eight-valuestoring system.

What is claimed is:
 1. A method of performing a reading operation in anonvolatile semiconductor memory device, the nonvolatile semiconductormemory device including a memory cell array which comprises: a memorystring having a plurality of memory cells connected in series therein; afirst select transistor connected to one end of the memory string; asecond select transistor connected to the other end of the memorystring; a bit line connected to the memory string via the first selecttransistor; a source line connected to the memory string via the secondselect transistor; and a word line connected to a control gate electrodeof each of the memory cells, the method of performing a readingoperation being configured to apply a reading voltage to the controlgate electrode of a selected memory cell in the memory string todetermine whether the selected memory cell is conductive or not, andthereby execute data reading of the selected memory cell, the methodcomprising: performing a first reading operation configured to set avoltage between the control gate electrode and a source of the selectedmemory cell to a first value and thereby read a threshold voltage set inthe selected memory cell; and performing a second reading operationconfigured to set the voltage between the control gate electrode and thesource of the selected memory cell to a voltage value which is smallerthan the first value, while keeping a voltage of the control gateelectrode of the selected memory cell at 0 or a positive value, andthereby read again the threshold voltage set in the selected memorycell.
 2. The method of performing a reading operation in a nonvolatilesemiconductor memory device according to claim 1, wherein the secondreading operation is performed by applying to the source line a sourceline voltage which is a positive voltage value.
 3. The method ofperforming a reading operation in a nonvolatile semiconductor memorydevice according to claim 2, wherein the second reading operation isperformed by applying a well voltage to a well where the memory cellsare formed, the well voltage having a voltage value which is lower thanthat of the source line voltage.
 4. The method of performing a readingoperation in a nonvolatile semiconductor memory device according toclaim 1, wherein each of the first reading operation and the secondreading operation is performed by applying to the source line a sourceline voltage which is a positive voltage value.
 5. The method ofperforming a reading operation in a nonvolatile semiconductor memorydevice according to claim 1, wherein in the second reading operation,the threshold voltage set in the selected memory cell is read again byselecting, as a voltage value between the control gate electrode and thesource of the selected memory cell, one of a plurality of voltage valuesthat are each smaller than the first value and different from eachother.
 6. The method of performing a reading operation in a nonvolatilesemiconductor memory device according to claim 1, wherein the secondreading operation is performed when it is determined that correctreading is impossible by the first reading operation.
 7. The method ofperforming a reading operation in a nonvolatile semiconductor memorydevice according to claim 1, wherein the second reading operation isperformed when a result of the first reading operation cannot beerror-corrected by ECC.
 8. A method of performing a reading operation ina nonvolatile semiconductor memory device, the nonvolatile semiconductormemory device including a memory cell array which comprises: a memorystring having a plurality of memory cells connected in series therein; afirst select transistor connected to one end of the memory string; asecond select transistor connected to the other end of the memorystring; a bit line connected to the memory string via the first selecttransistor; a source line connected to the memory string via the secondselect transistor; and a word line connected to a control gate electrodeof each of the memory cells, the method of performing a readingoperation being configured to apply a reading voltage to the controlgate electrode of a selected memory cell in the memory string todetermine whether the selected memory cell is conductive or not, andthereby execute data reading of the selected memory cell, the methodcomprising: performing a first reading operation configured to set avoltage between the control gate electrode and a source of the selectedmemory cell to a first value and thereby read a threshold voltage set inthe selected memory cell; and performing a second reading operationconfigured to set the voltage between the control gate electrode and thesource of the selected memory cell to a second value which is smallerthan the first value, while keeping a voltage of the control gateelectrode of the selected memory cell at 0 or a positive value, andthereby read again the threshold voltage set in the selected memorycell, or set the voltage between the control gate electrode and thesource of the selected memory cell to a third value which is smallerthan the second value, while keeping a voltage of the control gateelectrode of the selected memory cell at 0 or a positive value, andthereby read again the threshold voltage set in the selected memorycell.
 9. The method of performing a reading operation in a nonvolatilesemiconductor memory device according to claim 8, wherein the secondreading operation is performed by applying to the source line a sourceline voltage which is a positive voltage value.
 10. The method ofperforming a reading operation in a nonvolatile semiconductor memorydevice according to claim 9, wherein the second reading operation isperformed by applying a well voltage to a well where the memory cellsare formed, the well voltage having a voltage value which is lower thanthat of the source line voltage.
 11. The method of performing a readingoperation in a nonvolatile semiconductor memory device according toclaim 8, wherein each of the first reading operation and the secondreading operation is performed by applying to the source line a sourceline voltage which is a positive voltage value.
 12. The method ofperforming a reading operation in a nonvolatile semiconductor memorydevice according to claim 8, wherein the second reading operation isperformed when it is determined that correct reading is impossible bythe first reading operation.
 13. The method of performing a readingoperation in a nonvolatile semiconductor memory device according toclaim 8, wherein the second reading operation is performed when a resultof the first reading operation cannot be error-corrected by ECC.
 14. Amethod of performing a reading operation in a nonvolatile semiconductormemory device, the nonvolatile semiconductor memory device including amemory cell array which comprises: a memory string having a plurality ofmemory cells connected in series therein, each of the memory cells beingconfigured to be capable of having therein a first threshold voltagedistribution, a second threshold voltage distribution, a third thresholdvoltage distribution and a fourth threshold voltage distribution thatare assigned to data of four values to be stored in each of the memorycells and that have values ascending in an order of the first, second,third and fourth threshold voltage distributions; a first selecttransistor connected to one end of the memory string; a second selecttransistor connected to the other end of the memory string; a bit lineconnected to the memory string via the first select transistor; a sourceline connected to the memory string via the second select transistor;and a word line connected to a control gate electrode of each of thememory cells, the method of performing a reading operation beingconfigured to apply a reading voltage to the control gate electrode of aselected memory cell in the memory string to determine whether theselected memory cell is conductive or not, and thereby execute datareading of the selected memory cell, the method comprising: performing afirst reading operation configured to read a threshold voltage set inthe selected memory cell, the first reading operation including settinga voltage between the control gate electrode and a source of theselected memory cell to a first value and thereby determining whetherthe threshold voltage of the selected memory cell is in a range withinthe second threshold voltage distribution and over the second thresholdvoltage distribution; and performing a second reading operationconfigured to read again the threshold voltage set in the selectedmemory cell, the second reading operation including setting the voltagebetween the control gate electrode and the source of the selected memorycell to a voltage value which is smaller than the first value, whilekeeping a voltage of the control gate electrode of the selected memorycell at 0 or a positive value, and thereby determining whether thethreshold voltage of the selected memory cell is in the range within thesecond threshold voltage distribution and over the second thresholdvoltage distribution.
 15. The method of performing a reading operationin a nonvolatile semiconductor memory device according to claim 14,wherein the second reading operation is performed by applying to thesource line a source line voltage which is a positive voltage value. 16.The method of performing a reading operation in a nonvolatilesemiconductor memory device according to claim 15, wherein the secondreading operation is performed by applying a well voltage to a wellwhere the memory cells are formed, the well voltage having a voltagevalue which is lower than that of the source line voltage.
 17. Themethod of performing a reading operation in a nonvolatile semiconductormemory device according to claim 14, wherein each of the first readingoperation and the second reading operation is performed by applying tothe source line a source line voltage which is a positive voltage value.18. The method of performing a reading operation in a nonvolatilesemiconductor memory device according to claim 14, wherein in the secondreading operation, it is determined whether the threshold voltage of theselected memory cell is in the range within the second threshold voltagedistribution and over the second threshold voltage distribution byselecting, as a voltage value between the control gate electrode and thesource of the selected memory cell, one of a plurality of voltage valuesthat are each smaller than the first value and different from eachother.
 19. The method of performing a reading operation in a nonvolatilesemiconductor memory device according to claim 14, wherein the secondreading operation includes determining whether the threshold voltage ofthe selected memory cell is in the range within the second thresholdvoltage distribution and over the second threshold voltage distributionby setting a reading voltage applied to the control gate electrode to afirst reading voltage value, and, moreover, determining whether thethreshold voltage of the selected memory cell is in a range within thethird threshold voltage distribution and over the third thresholdvoltage distribution by setting a reading voltage applied to the controlgate electrode to a second reading voltage value which is larger thanthe first reading voltage value, and determining whether the thresholdvoltage of the selected memory cell is in a range within the fourththreshold voltage distribution and over the fourth threshold voltagedistribution by setting a reading voltage applied to the control gateelectrode to a third reading voltage value which is larger than thesecond reading voltage value.
 20. The method of performing a readingoperation in a nonvolatile semiconductor memory device according toclaim 19, wherein the first reading operation includes determiningwhether the threshold voltage of the selected memory cell is in therange within the third threshold voltage distribution and over the thirdthreshold voltage distribution by applying to the control gate electrodea reading voltage which is larger than the second reading voltage value,and determining whether the threshold voltage of the selected memorycell is in the range within the fourth threshold voltage distributionand over the fourth threshold voltage distribution by applying to thecontrol gate electrode a reading voltage which is larger than the thirdreading voltage value.